This present invention relates to an angle measuring device operating without a graduated board.
An example of an angle measuring device operating without a graduated board is disclosed in Report ETL-TR-1 (1972.1) of the U.S. Army Engineer Topographic Laboratories and is well known in the art. FIG. 1 is a plan view of the device, and FIG. 2 is a sectional view taken along a line II--II in FIG. 1. In this device, a rotary disc 10 together with shafts 11 and 12 and a base 13 is rotated by a motor (not shown). The rotary disc 10 has a slit 15. In FIG. 1, reference character O designates the center of rotation of the disc.
A first reference point 21 is fixedly provided on a housing (not shown). A light-emitting element 21a and a light-sensing element 21b are fixedly provided at the position of the first reference point on the housing on both sides of the disc 10. The slit 15 passes between the elements 21a and 21b while the disc is rotating. A second reference point 22 is provided on the housing with the points 21 and 22 being positioned symmetrically with respect to the center of rotation O. A light-emitting element 22a and a light-sensing element 22b are disposed at the position of the second reference point 22 on the housing with the elements 22a and 22b being on both sides of the disc 10 and with the slit 15 passing between the elements 22a and 22b while the disc is rotating.
An angle measuring arm 30, rotable around the point O, has a first measuring point 31 at one end. A light-emitting element 31a and a light-sensing element 31b are fixedly provided at the position of the first measuring point 31 on the arm 30 on both sides of the disc 10. The slit 15 passes between the elements 31a and 31b while the disc is rotating. A second measuring point 32 is provided on the arm 30 is such a manner that the first and second measuring points 31 and 32 are positioned symmetrically with respect to the center of rotation O. A light-emitting element 32a and a light-sensing element 32b are fixedly provided at the position of the second measuring point on the arm 30. The slit 15 passes between the elements 32a and 32b while the disc 10 is rotating.
In FIG. 1, an angle A is formed by a straight line passing through the first reference point 21 and the center of rotation O and a straight line passing through the first measuring point 31 and the center of rotation O, and an angle B is formed by a straight line passing through the second reference point 22 and the center of rotation O and a straight line passing through the second measuring point 32 and the center of rotation O.
An angle between two objects can be measured with the above-described device as follows: First, the center O is made to coincide with the position where the angle is to be measured. Then, the first reference point 21 is so set that the first object is on an extension of line passing through the point O and the first reference point 21. Thereafter, the first measuring point 31 is so set that the second object is on an extension of a line passing through the point O and the first measuring point 31. In other words, the arm 30 is turned so that the second object is on the extension of the line passing through the point O and the point 31.
FIG. 3 is a block diagram showing a signal processing circuit used in the above-described angle measuring device. An angle measuring counter 41 counts the output pulses from an oscillator circuit 43 for a period of time which elapses from the time instant that it receives the output signal of the first reference point light-sensing element 21b until it receives the output signal of the first measuring point light-sensing element 31b. A one-revoution counter 42 counts the output pulses of the oscillator circuit 43 for a period of time which elapses from the time instant that it receives one output signal of the first reference point light-sensing element 21b until it receives the next output signal of the same element 21b. In an arithmetic circuit 44, the ratio of the number of pulses counted by the counter 41 to the number of pulses counted by the counter 42 is multiplied by 360.degree. to calculate the angle A. While the disc 10 makes a first revolution, the counters 41 and 42 operate as described above, and while the disc 10 makes the next revolution, the arithmetic circuit 44 performs the above-described calculations. This operation is repeatedly carried out. In other words, the counting operations are carried out for odd-numbered revolutions of the disc 10, and the calculations are carried out for even-numbered revolutions of the disc 10. When the last counting operation and the last calculation have been achieved, the resultant data points B are averaged.
However, sometimes the center of rotation of the disc 10 may not coincide with the center of rotation of the arm 30. In this case, a measuring angle error (eccentric error) arises. In order to correct such eccentric error, a so-called "180.degree. opposed reading method" is employed. In accordance with that method, the angle A is measured by using the first reference point 21 and the first measuring point 31 as described above, similarly the angle B is measured by using the second reference point 22 and the second measuring point 32 which are respectively opposed to the first reference point 21 and the first measuring point 31, and the average value of the angles A and B is obtained to correct the eccentric error.
The above-described method suffers from a drawback that a relatively long time is required for the measurment. During one revolution of the disc 10 the angle measuring counter 41 and the one-revolution counter 42 count the numbers of pulses corresponding to given angles, and during the next revolution of the disc 10 the arithmetic circuit 44, receiving the outputs of these counters, performs the above-described calculations. That is, in order to measure the angle A once, it is necessary to cause the disc 10 to make two revolutions. If the above-described cycle is repeated ten times with the stability of the motor used to rotate the disc 10 taken into account, then 20 revolutions of the disc 10 are required. If the above-described operation is performed for the angle B also to correct for the eccentric error, it is required to cause the disc 10 to make 40 revolutions. Thus, in the conventional method, the time required for angle measurement is relatively long.
In view of the above-described difficulties accompanying a conventional angle measuring device, an object of the invention is to provide an angle measuring device operating without a graduated board in which the time required for angle measurement is relatively short.
Further, the above-described conventional device suffers from a problem that sometimes the measured angle is incorrect. This will be described in more detail. In order to measure the angle A, the disc 10 is rotated. More specifically, the angle is calculated from a time required for the slit 15 to reach the light-sensing element 31b from the light-sensing element 21b during the rotation of the disc 10. Accordingly, in the case where the angle A is near 0.degree., the slit 15 passes the light-sensing element 31b immediately after it has passed the light-sensing element 21b, and therefore the output pulse of the light-sensing element 31b should rise immediately after the output pulse of the light-sensing element 21 has risen. However, in practice, sometimes the order of rising of the two signals is reversed because of small variation of the two pulses. In such a case, the angle A is determined to be about 360.degree. upon processing the signals. On the other hand, although an angle may be near 360.degree., it may be determined to be about 0.degree.. Thus, not only when an angle of about 0.degree. is measured will the measured value be incorrect as described above, but also when an angle is repeatedly measured to obtain the average value, the result may be incorrect.
In view of the foregoing, another object of the invention is to provide an angle measuring device operating without a graduated board, in which, when an angle of about 0.degree. is measured, the results of the measurement is correct.
Further, in the above-described conventional device, an angle .theta. is formed between a straight line passing through the reference point 21 and the central point O and a straight line passing through the measuring point 31 and the central point O.
The above-mentioned Report discloses the study of an eccentric error (which is an angle measurement error due to the fact that the center of rotation of the disc 10 deviates from the center of rotation of the angle measuring arm 30) and described a so-called "180.degree. opposed reading method" used to correct the eccentric error. However, even if such a method is employed, the eccentric error may still not be sufficiently corrected.
FIG. 4 is a timing chart for the above-described conventional angle measuring device. When the slit 15 of the disc 10 passes through the reference point 21, a reference point pulse is produced by the light-sensing element 21b, and when the slit 15 passes through the measuring point 31, a measuring point pulse is generated by the light-sensing element 31b. A period of time between the rise of the reference point pulse and the rise of the measuring point pulse is referred to as a first time T.sub.1, and a period of time between the rise of a reference point pulse and the rise of the next reference point pulse, i.e., a period of time for the disc 10 to make one complete revolution, is referred to as a one-revolution time T.sub.3.
In the above-described conventional device, the angle .theta. can be obtained from the following expression: EQU .theta. (degree)=(T.sub.1 /T.sub.3).times.360.degree..
However, the conventional device suffers from a drawback in that when the center of the disc 10 is shifted from the center of rotation of the angle measuring arm 30, the measured angle value includes an eccentric error. In case where the center of the disc 10 is shifted from the center of rotation of the arm 30, the large area of the light-sensing element 31b receives the light beam from the light-sensing element 31a when the slit 15 comes closest to the light-sensing element 31a as shown in FIG. 5A, and the small area of the light-sensing element 31b receives the light beam when the slit 15 comes closest to the light-sensing element 31b as shown in FIG. 5B. Accordingly, when the center of the disc 10 is not shifted from the center of rotation of the arm 30, the waveforms of the pulses are as indicated by the solid lines in FIG. 4. However, when the center of the disc 10 is shifted from the center of rotation of the arm 30, the waveforms of the pulses are as indicated by the broken lines in FIG. 4.
As is apparent from FIG. 4, the first time T.sub.1 in the case where the center of the disc 10 is not shifted from the center of rotation of the arm 30 is different from the first time T.sub.1 +.alpha. in the case where the center of the disc 10 is shifted from the center of rotation of the arm. Thus, in the latter case, the measured angle value includes an eccentric error.
In other words, in the case where the center of the disc 10 does not coincide with the center of rotation of the angle measuring arm, depending on the position of the arm on the circumference, the position of passage of the slit 15 is changed in the optical path from the light-sensing element 31a to the light-sensing element 31b (the position of the slit 15 in FIG. 5A being different from that of the slit 15 in FIG. 5B), and therefore the output of the light-sensing element 31 is changed. Because of this phenomenon, in the conventional device, eccentric error is not sufficiently corrected.
In view of the foregoing, another object of the invention is the provision of an angle measuring device operating without a graduated board in which, even when the center of the rotary disc is shifted from the center of rotation of the angle measuring arm, the measured angle includes no eccentric error.